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<title>Issue 3985: ranges::to should Mandates C not to be view</title>
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<p><em>This page is a snapshot from the LWG issues list, see the <a href="lwg-active.html">Library Active Issues List</a> for more information and the meaning of <a href="lwg-active.html#New">New</a> status.</em></p>
<h3 id="3985"><a href="lwg-active.html#3985">3985</a>. <code>ranges::to</code> should <i>Mandates</i> <code>C</code> not to be view</h3>
<p><b>Section:</b> 25.5.7.2 <a href="https://wg21.link/range.utility.conv.to">[range.utility.conv.to]</a>, 25.5.7.3 <a href="https://wg21.link/range.utility.conv.adaptors">[range.utility.conv.adaptors]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2023-08-25 <b>Last modified:</b> 2023-10-30</p>
<p><b>Priority: </b>3
</p>
<p><b>View other</b> <a href="lwg-index-open.html#range.utility.conv.to">active issues</a> in [range.utility.conv.to].</p>
<p><b>View all other</b> <a href="lwg-index.html#range.utility.conv.to">issues</a> in [range.utility.conv.to].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
In order to solve issues in LWG <a href="lwg-defects.html#3787" title="ranges::to's template parameter C should not be a reference type (Status: Resolved)">3787</a><sup><a href="https://cplusplus.github.io/LWG/issue3787" title="Latest snapshot">(i)</a></sup> and LWG <a href="lwg-defects.html#3847" title="ranges::to can still return views (Status: C++23)">3847</a><sup><a href="https://cplusplus.github.io/LWG/issue3847" title="Latest snapshot">(i)</a></sup> that the template parameter <code>C</code> 
can be specified as an unreasonable type, <code>ranges::to</code> adds a <i>Mandates</i> that requires <code>C</code> to be a 
<i>cv</i>-unqualified class type.
</p>
<p>
However, the earliest requirement that <code>C</code> not be a view was still imposed by the constraints of function signatures,
although it is unclear why <i>Constraints</i> were used in the first place, such a way of kicking out the function candidate 
does bring undesirable hard errors and poor diagnostics (<a href="https://godbolt.org/z/K1f15K71n">demo</a>):
</p>
<blockquote><pre>
#include &lt;ranges&gt;

int main() {
  auto iota = std::views::iota(0, 10);
  auto take = std::ranges::to&lt;std::ranges::take_view&gt;(iota, 5);  // <span style="color:red;font-weight:bolder">hard error in function body</span>
  auto drop = iota | std::ranges::to&lt;std::ranges::drop_view&gt;(5); // <span style="color:red;font-weight:bolder">poor diagnostics</span>
}
</pre></blockquote>
<p>
I think consistent use of <i>Mandates</i> for template parameter <code>C</code> is more appropriate, as <code>static_assert</code> 
provide clearer and more readable diagnostics.
</p>

<p><i>[2023-10-30; Reflector poll]</i></p>

<p>
Set priority to 3 after reflector poll.
</p>



<p id="res-3985"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4958" title=" Working Draft, Programming Languages — C++">N4958</a>.
</p>

<ol>

<li><p>Modify 25.2 <a href="https://wg21.link/ranges.syn">[ranges.syn]</a> as indicated:</p>

<blockquote>
<pre>
#include &lt;compare&gt;              // <i>see 17.12.1 <a href="https://wg21.link/compare.syn">[compare.syn]</a></i>
#include &lt;initializer_list&gt;     // <i>see 17.11.2 <a href="https://wg21.link/initializer.list.syn">[initializer.list.syn]</a></i>
#include &lt;iterator&gt;             // <i>see 24.2 <a href="https://wg21.link/iterator.synopsis">[iterator.synopsis]</a></i>

namespace std::ranges {
  [&hellip;]
  // <i>25.5.7 <a href="https://wg21.link/range.utility.conv">[range.utility.conv]</a>, range conversions</i>
  template&lt;class C, input_range R, class... Args&gt; <del>requires (!view&lt;C&gt;)</del>
    constexpr C to(R&amp;&amp; r, Args&amp;&amp;... args);                                          // <i>freestanding</i>
  template&lt;template&lt;class...&gt; class C, input_range R, class... Args&gt;
    constexpr auto to(R&amp;&amp; r, Args&amp;&amp;... args);                                       // <i>freestanding</i>
  template&lt;class C, class... Args&gt; <del>requires (!view&lt;C&gt;)</del>
    constexpr auto to(Args&amp;&amp;... args);                                              // <i>freestanding</i>
  template&lt;template&lt;class...&gt; class C, class... Args&gt;
    constexpr auto to(Args&amp;&amp;... args);                                              // <i>freestanding</i>
  [&hellip;]
}
</pre>
</blockquote>

</li>

<li><p>Modify 25.5.7.2 <a href="https://wg21.link/range.utility.conv.to">[range.utility.conv.to]</a> as indicated:</p>

<blockquote>
<pre>template&lt;class C, input_range R, class... Args&gt; <del>requires (!view&lt;C&gt;)</del>
  constexpr C to(R&amp;&amp; r, Args&amp;&amp;... args);
</pre>
<blockquote>
<p>
-1- <i>Mandates</i>: <code>C</code> is a cv-unqualified class type <ins>and does not satisfy <code>view</code></ins>.
</p>
[&hellip;]
</blockquote>
</blockquote>

</li>

<li><p>Modify 25.5.7.3 <a href="https://wg21.link/range.utility.conv.adaptors">[range.utility.conv.adaptors]</a> as indicated:</p>

<blockquote>
<pre>template&lt;class C, class... Args&gt; <del>requires (!view&lt;C&gt;)</del>
  constexpr auto to(Args&amp;&amp;... args);
template&lt;template&lt;class...&gt; class C, class... Args&gt;
  constexpr auto to(Args&amp;&amp;... args);
</pre>
<blockquote>
<p>
-1- <i>Mandates</i>: For the first overload, <code>C</code> is a cv-unqualified class type <ins>and does not satisfy <code>view</code></ins>.
</p>
[&hellip;]
</blockquote>
</blockquote>

</li>

</ol>





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